In various types of cryogenic refrigerators, or cryopumps, a working fluid such as helium is introduced into a cylinder, and the fluid is expanded at one end of a piston-like displacer to cool the cylinder. For example, in Gifford-McMahon type refrigerators working fluid under high pressure may be valved into the warm end of the cylinder. Then the fluid is passed through a regenerator by movement of the displacer. The fluid which has been cooled in the regenerator is then expanded at the cold end of the displacer.
The displacer movement may be controlled by a mechanical drive such as a rotary synchronous motor which drives the displacer through a rotary to linear crosshead. The crosshead converts the rotary motion of the motor to reciprocating motion which drives the displacer through a sinusoidal displacement cycle. The displacer may also be driven by a linear motor which directly drives the displacer though the displacement cycle.
Sufficient mechanical torque must be generated by the drive motor to drive the displacer through each refrigeration cycle. The instantaneous torque load on the motor depends on the refrigeration cycle and the mechanical condition of the refrigerator. If the instantaneous torque load on a synchronous motor reaches some threshold, i.e., the maximum attainable torque, the motor loses synchronization and pulls-out or "ratchets".
As the helium refrigerant in the system becomes contaminated through leakage and the like, there is an increased torque load on the synchronous motor. Mechanical wear of the cryogenic refrigerator components also tend to increase the torque load on the motor. Thus, with refrigerant contamination and mechanical wear the peak torque load on the motor increases. When the peak torque increases beyond the maximum attainable torque of the synchronous motor, the synchronous motor loses synchronization until the system moves into a lower region of the torque cycle, i.e., the motor is overloaded.